Coupling agents are known to act as an adhesive between inorganic surfaces and polymers. For example, organofunctional silane coupling agents act as adhesion promoters between organic polymers and mineral surfaces. Less well known but also effective coupling agents include chromium complexes which are effective when used in combination with either silica or alumina surfaces.
While such coupling agents are most frequently used as adhesion primer layers between glass and polymeric films, they are also well known in coating technology. Typical paper and polymeric web coatings often contain combinations of polymers and inorganic particles such as silica. In order to ensure compatibility and adhesion between the polymeric matrix and silica particle, silane coupling agents or silanol modified polymers are sometimes used. In U.S. Pat. No. 5,562,975 (New Oji Paper Co., Ltd), a hot melt ink recording sheet is disclosed in which the ink receptive layer comprises a mixture of silanol-modified polyvinyl alcohol and amorphous silica. The '975 patent states: "The silanol groups of the modified polyvinyl alcohol chemically react with the pigment so as to enhance the mechanical strength of the ink-receiving layer". For the porous, non-glossy coatings described therein, there is a surfeit of silica particles such that it is safe to assume that by intention and in practice virtually all the silanol groups on the polyvinyl alcohol are fully reacted in the dried coating and would no longer be available to react with components of the printing ink.
Similarly, in EP 0 759 365 (New Oji Paper Co., Ltd), an inkjet recording material intended for use as stickers is disclosed. In this case, the silica particles are defined as either porous amorphous silica particles if a non-glossy film is desired, or colloidal silica particles if a glossier film is desired. Effective binder systems include either conventional polyvinyl alcohol or silicon-modified polyvinyl alcohol. Again, silanol groups on the polyvinyl alcohol react with silica surfaces in the coating, and are not available to react with ink components.
A somewhat different approach for use of silane coupling agents in ink receiving layers is described in U.S. Pat. No. 5,352,736 (Minnesota Mining and Manufacturing Company). In this case, a semi-interpenetrating network of a water soluble polymer and a non-water soluble polymer form the recording layer. The non-water soluble portion of the layer contains crosslinked silanol moieties which originate from added silane coupling agents. Once again, the reactive species are fully crosslinked in the dried film and so can not be available for reacting with ink species.
While no record exists of reactive ink-receiver combinations using chemistries described herein, some reactive combinations have been disclosed which are summarized below.
In U.S. Pat. No. 4,649,064 (Eastman Kodak Company), ink-receiver combinations are disclosed in which the ink contains a cross-linkable colorant/resin composition and the receiver layer contains a cross-linking agent which renders the printed areas abrasion-resistant, smear-resistant and waterfast. In this case, the reactive substance in the ink is a polymeric resin. While such an approach is somewhat effective, polymeric resin additives typically have the unfortunate characteristic of clogging ink nozzles which is not observed for inorganic colloidal additives to inks.
A similar approach is disclosed in U.S. Pat. No. 5,537,137 and EP 0 775 596 (both to E. I. DuPont de Nemours and Company) in which a reactive component is contained in the ink receiving layer. The reactive species is capable of crosslinking the coating itself as well as binding the colorant to the coating. The reactive species must be chosen so that it does not react until it is exposed to an external energy source such as heat or radiation after printing. Such an approach has distinct disadvantages due to the inconvenience of a required post-printing treatment.
Inks containing inorganic particles have been disclosed for inkjet printing applications. In U.S. Pat. No. 5,221,332 (Xerox Corporation), dye based inks containing silica particles are disclosed. The silica particles in the '332 patent are intended to enable increased drop volumes while printing. Combination of these inks with a specially designed receiver is not taught.
Copending, commonly owned, concurrently filed U.S. application Ser. No. 09/126,866 of Martin and Bugner, titled "PIGMENTED INK JET INKS CONTAINING ALUMINUM STABILIZED COLLOIDAL SILICA", now U.S. Pat. No. 5,925,178, describes ink additives comprising preferably aluminium stabilized colloidal silica in order to improve optical density and dry abrasion resistance.
There is, however, still a need for other mechanisms to further enhance water resistance and durability of images printed with inks containing inorganic particles.